Transition structure for broadband coupling of dielectric rod antenna to coaxial feed



June 30, 1970 R. H. HALLENDORFF TRANSITION STRUCTURE FOR BROADBAND COUPLING OF DIELECTRIC ROD ANTENNA '10 COAXIAL FEED Filed April 23. 1968 RICHARD H. HALLENDORFF ATT NEY United States Patent U.S. Cl. 343-785 7 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a so-called current loop transition structure for exciting a TE mode in a double ridge, dielectrically loaded circular waveguide from a colinear, end-fire coaxial system, for application to a waveguide or dielectric rod antenna system. Generally speaking, the transition structure of the present invention comprises: a pair of tapered metallic members which mate with similarly tapered slots in the dielectric rod member of the waveguide; a central conductor member which is connected to the central conductor of the coaxial system and which has a substantially 90 bend at one end, for connection to one of the tapered metallic members and so that it abuts the dielectric rod member of the waveguide at the center thereof; and, a cylindrical conductive housing member which surrounds the dielectric rod member, in contact with the tapered metallic members, and forms the outer wall of the waveguide. The proposed structure is also designed such that the dielectric rod member is securely held in place and is therefore particularly adapted for antenna use on high speed missiles, for example.

BACKGROUND OF THE INVENTION Present day high speed missiles often employ a ceramic rod antenna system mounted on the front of the missile, with the ceramic rod tapered so as to conform electrically and provide aerodynamic streamlining. Such antenna system must also be of rugged construction so that it will not change its operating characteristics or become dis lodged during missile flight. Another of the existing problems with regard to such colinear feed dielectric rod antenna systems is that it has heretofore been quite diflicult to maintain the proper impedance matching necessary -to permit broadband operation of the antenna system.

DESCRIPTION OF THE INVENTION In view of the foregoing, it is proposed in accordance with the present invention to provide a transition structure capable of exciting a TE mode in a double ridge, dielectrically loaded circular waveguide from a colinear, end-fire coaxial system, for application to a waveguide or dielectric rod antenna system such as that previously described as being used on present day high speed missiles. More specifically, the proposed structure of the present invention improves upon the related prior art in that it permits broadband operation of the antenna system over a 30%40% frequency band with low frequency operation relatively close to normal waveguide cut-off frequency; it reduces impedance sensitivity in the vicinity of the transition; and, it permits design of a dielectric rod antenna structure with minimum frontal area for critical mounting configurations. Moreover, the proposed transition structure is such that the antenna is securely held in place longitudinally as well as axially, as verified by high speed missile flight tests.

3,518,691 Patented June 30, 1970 Other objects, purposes and characteristic features of the present invention will in part be pointed out as the description of the present invention progresses and in part be obvious from the accompanying drawings wherein:

FIG. 1 is an exploded perspective view of a dielectric rod antenna system utilizing the transition structure of the present invention; and

FIG. 2 is an enlarged, cross-sectional view of the proposed transition structure of the present invention, in its assembled condition.

Referring now to the preferred embodiment of FIG. 1, the proposed transition structure is illustrated as being used for exciting a TE mode in a dielectric (for example, ceramic) rod antenna element 10 which is tapered for RF radiation and aerodynamic streamlining. As mentioned previously, such antenna element configuration is essential when the rod antenna is mounted externally on the nose of a high speed missile. The left-hand or base end of the rod element 10 has a slightly larger diameter than the central portion of the rod element, so as to form a locking shoulder 11, and is configured with diametrically opposed tapered slots, represented at 12 and 13 respectively. These tapered slots 12 and 13 terminate at a centrally located, transverse edge 14 whose purpose will be described in more detail hereinafter.

A pair of tapered metallic members 15 and 16 which might, for example, be made of steel are adapted to mate in the tapered slots 12 and 13, as shown in FIG. 2. These tapered members 15 and 16 function to impedance match the low impedance of the dielectric rod antenna. This form of impedance matching structure is also quite stable at elevated temperatures encountered in high speed missile flight since the tapered section impedance transformer is relatively insensitive. Moreover, the tapered members 15 and 16 serve to prevent rotation of the rod antenna during flight.

A central conductor member 17, adapted to be connected to the center conductor of the input coaxial sys tem, is formed with a substantially bend at its extending right-hand end and is received in a small bore in the lower tapered member 16, to form a shorted current loop. The bent end of conductor member 17 abuts against the transverse edge 14 on the rod antenna element 10 when the transition structure is assembled, as shown in FIG. 2.

A metallic cylindrical housing member 18 has an inside diameter substantially equal to that of the enlarged base portion of the rod antenna element 10 and is adapted to receive such base portion, together with the tapered insert members 15 and 16, in the assembled condition illustrated in FIG. 2. Moreover, housing member 18 is threaded at its right-hand end to receive a locking collar 19 which, in turn, is provided with a retaining shoulder portion 20 which engages the locking shoulder 11 on the base end of antenna element 10 and thereby retains the antenna element 10 securely in assembled position. Four small screws 21 extend through countersunk holes in the side walls of the cylindrical housing member 18 and are received in threaded holes 22 in the metallic tapered members 15 and 16. The cylindrical housing member 18 is adapted to be connected, for example at the reduced body portion 23, to the outer conductor of the input coaxial system.

Referring more particularly to FIG. 2, when the transition structure of the present invention is in its assembled position, the dielectric rod element 10 is held securely in place by the retaining collar 19. Moreover, the metallic tapered members 15 and 16 prevent rotation of the rod element 10 within the housing member 18. As also illustrated in FIG. 2, the metallic members 15 and 16 abut the inside wall 24 of the housing member 18, at the formation of reduced body portion 23, and are dimensioned to start out flush or coextensive with the inside diameter of the reduced body portion 23 of housing member 18 and then gradually taper to smoothly conform with the tapered slots 12 and 13 in the base portion of dielectric rod element 10. Accordingly, a smooth impedance variation from the input coaxial system to the dielectric rod antenna is accomplished, to thereby permit broadband operation of the dielectric rod antenna system very near to normal waveguide cut-ofi frequency and a corresponding reduction in the diameter of the rod antenna. In the past, this has been ditficult to attain, especially where a shorted current loop, such as is formed by center conductor 17, is employed to generate a TE mode in the waveguide portion of the transition structure.

Although the foregoing description has dealt primarily with rod antenna application of the proposed transition structure, it should be obvious that the proposed transition structure is equally adapted for exciting a TE mode in a dielectrically loaded circular 'waveguide. Here again, the double tapered ridge configuration of the transition structure would serve to provide proper impedance matching of the input coaxial system to the waveguide and thereby permit broadband operation very near to the waveguide cut-off frequency.

Other modifications, adaptations and alterations of the present invention are possible in the light of the above teachings. Therefore, it should be understood that, within the spirit and scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A colinear transition structure for exciting a TE mode in a dielectric rod antenna or dielectrically loaded circular waveguide from an input coaxial system comprising:

a pair of tapered electrically conductive members;

said antenna or waveguide including a dielectric rodlike member formed at one end with diametrically opposed tapered slots adapted to receive said tapered members;

said slots extending longitudinally along said dielectric member with the depth of said slots decreasing in a direction away from said one end of said dielectric member;

an electrically conductive, cylindrical housing member adapted to encircle said one end of said dielectric rod-like member in contact with said tapered conductive members and to be connected to the outer conductor of said coaxial system, and

a central conductor disposed to extend coaxially within said housing member and adapted to be connected to the inner conductor of said coaxial system,

the extending end of said central conductor having a substantially bend and being affixed to one of said tapered conductive members and abutting said dielectric member substantially at the center thereof.

2. The colinear transition structure specified in claim 1, wherein:

the tapered slots in said dielectric member terminate in a transverse edge spaced longitudinally from said one end of said dielectric member, at substantially the cross-sectional center of said dielectric member, and wherein the bent extending end of said central conductor abuts against said transverse edge when said tapered conductive members are mated in said tapered slots.

3. The colinear transition structure specified in claim 1, wherein said tapered conductive members are steel.

4. The colinear transition structure specified in claim 1, wherein said dielectric member is a ceramic rod antenna element.

5. The colinear transition structure specified in claim 4, wherein:

the diameter of said slotted end of said ceramic rod antenna element being larger than the remainder thereof to form a locking shoulder, and further including means cooperating with said locking shoulder for retaining said slotted end of said ceramic rod antenna element Within said cylindrical housing member.

6. The colinear transition structure specified in claim 5, wherein said retaining means is a locking collar adapted to be threadably received by said cylindrical housing member and being formed with a shoulder which engages said locking shoulder on said ceramic rod antenna element.

7. The colinear transition structure specified in claim 1, wherein:

said cylindrical housing member has a reduced diameter body portion formed thereon adapted to be connected to the outer conductor of said input coaxial system, and wherein said tapered conductive members abut the inside wall of said housing member at the formation of said reduced body portion and are dimensioned to be there coextensive with the inside diameter of said reduced body portion.

References Cited UNITED STATES PATENTS 3,128,467 4/1964 Lanctot 343-785 ELI LIEBERMAN, Primary Examiner US. Cl. X.R. 

